Herodotus (490-425/420 BCE)
“A bow kept always strung would break, and so be useless when it was needed. It is the same with a man; anyone who was always serious, and never allowed himself a fair share of relaxation and amusement, would suddenly go off his head, or get a stroke.”
- From The Histories by Herodotus (Book II, section 173), translated by Aubrey De Sélincourt and revised by John Marincola (Penguin Classics, 2002).